1. Field of the Invention
This invention relates to capping stations for thermal ink jet printing apparatus and is concerned, more particularly, with providing a method to remove waste ink from capping stations on low cost integral capping, priming and wiping stations.
2. Description of Related Art
An ink jet printer of the so-called "drop on demand" type has at least one printhead from which droplets of ink are directed towards a recording medium. Within the printhead, the ink may be contained in a plurality of channels and energy pulses are used to cause the droplets of ink to be expelled, as required from orifices at the ends of the channels.
In a thermal ink jet printer, the energy pulses are usually produced by resistors, each located in a respective one of the channels, which are individually addressable by current pulses to heat and vaporize ink in the channels. As voltage is applied across a selected resistor, a vapor bubble grows in that particular channel and ink bulges from the channel orifices. At that stage, the bubble begins to collapse. The ink within the channel retracts and separates from the bulging ink which forms a droplet moving in a direction away from the channel orifice and towards the recording medium. The channel is then refilled by capillary action, which in turn draws ink from a supply container. Operation of a thermal ink jet printer is described in, for example, U.S. Pat. No. 4,849,774 to Endo et al.
One particular form of thermal ink jet printer is described in U.S. Pat. No. 4,638,337 to Torpey et al. That printer is of the carriage type and has a plurality of printheads, each with its own ink supply cartridge, mounted on a reciprocating carriage. The channel orifices in each printhead are aligned perpendicular to the line of movement of the carriage and a swath of information is printed on the stationary recording medium as the carriage is moved in one direction. The recording medium is then stepped, perpendicular to the line of carriage movement, by a distance equal to the width of the printed swath and the carriage is then moved in the reverse direction to print another swath of information.
It has been recognized that there is a need to maintain the ink ejecting orifices of an ink jet printer, for example, by periodically cleaning the orifices when the printer is in use, and/or by capping the printhead when a printer is not in use or is idle for extended periods. The capping of the printhead is intended to prevent the ink in the printhead from drying out. There is also a need to prime a printhead before use, to ensure the printhead channels are completely filled with ink and contain no contaminants or air bubbles. Maintenance and/or priming stations for the printheads of various types of ink jet printer are described in, for example, U.S. Pat. No. 4,745,414, to Okamura et al.; U.S. Pat. No. 4,855,764, to Humbs et al.; U.S. Pat. No. 4,853,717, to Harmon et al.; and U.S. Pat. No. 4,746,938, to Yamamori et al., while the removal of gas from the ink reservoir of a printhead during printing is described in U.S. Pat. No. 4,679,059, to Dagna. Other ink jet printer nozzle cleaning and sealing devices are described in, for example, U.S. Pat. No. 4,401,990, to Aiba et al.; U.S. Pat. No. 4,567,494, to Taylor; U.S. Pat. No. 5,055,856, to Tomii et al.; U.S. Pat. No. 4,935,753, to Lehmann et al.; U.S. Pat. No. 4,306,245, to Kasugayama et al.; and U.S. Pat. No. 5,040,000, to Yokoi.
It has been found that the printing operation or the priming operation, which usually involves either forcing or drawing ink through the printhead, can leave drops of ink on the face of the printhead and that, ultimately, there is a build-up of ink residue on the printhead face. That residue can have a deleterious effect on print quality. It has also been found that paper fibers and other foreign material can collect on the printhead face while printing is in progress, and like the ink residue, can also have a deleterious effect on print quality. It has previously been proposed, in U.S. Pat. No. 4,853,717, to Harmon et al., that a printhead should be moved across a wiper blade at the end of a printing operation so that paper dust and other contaminants are scraped off the orifice plate before the printhead is capped. In U.S. Pat. No. 4,745,414, to Okamura et al., it has been proposed to provide an ink jet printer with a motorized ink discharge recovery device that employs a suction pump and a wiping means in a one cycle sequence for preventing unsatisfactory ink discharge of a recording head. It has also been proposed, in U.S. Pat. No. 4,746,938, to Yamamori et al., that an ink jet printer should be provided with a washing unit which, at the end of a printing operation, directs water at the face of the printhead to clean the latter before it is capped.
A typical thermal ink jet printer, as shown in FIG. 1, has a printhead (with integral ink supply) 18 mounted on a printer fast scan carriage (scanning carriage) 19. The printhead 18 contains a plurality of ink channels which carry ink from the integral ink supply to respective ink ejecting orifices. In use, the scanning carriage 19 reciprocates, as indicated by arrow 19a, and droplets of ink are expelled by selected ones of the printhead orifices (such as in the manner previously described) and are directed towards a recording medium 69. During each pass of the scanning carriage 19 the recording medium 69 is stationary. At the end of each pass, the recording medium 69 is stepped up to the next print line in the direction of arrow 69a.
At one side of the typical thermal ink jet printer, outside the printing zone, which encompasses the width of the recording medium 69, is a typical maintenance station 71. At the end of the printing operation, the scanning carriage 19 is parked in a maintenance position confronting the maintenance station 71, which comprises a chamber 73 and an associated suction pump 75 in communication with each other through a waste tank 77 and lines 79, 81 interconnecting the interior of the chamber 73 with the waste tank 77 and the waste tank 77 and the pump 75, respectively. The chamber 73 is movable towards and away from the printhead 18, as indicated by the arrow 73a, by, for example, a solenoid 83, and has a seal means 85 secured to a rigid wall 87 of the chamber 73.
The typical maintenance station 71 then, must provide a humid environment for the printhead orifices or nozzles, and must also perform the function of priming the printhead 18. As is known in the inkjet printer art, the priming operation draws ink from the ink supply and fills the printhead channels and also clears out air trapped in the printhead channels. A low cost, low volume vacuum or suction pump capable of drawing a partial vacuum of 65 to 140 inches of water is common. A waste tank 77 collects waste ink during priming of the printhead 18.
The vacuum suction rises preferably in approximately two seconds to a point where the ink channels are well primed. When the typical maintenance station 71 is pulled away from the printhead 18 (as will be discussed in greater detail with respect to the inventive method described herein), air rushing in over the printhead 18 and maintenance station 71 clears any waste ink from the printhead face.
Further, another existing design of a simple priming and capping station has the problem of limited waste ink capacity which can result in print cartridge and machine contamination. This prior art design involves an integral capping and priming mechanism in which both the cap seal actuation and priming pump compression are provided by the motion of the scanning carriage (printhead holder). When the scanning carriage is scanned off the recording medium (e.g., paper) the scanning carriage encounters the capping station pawl which initially pushes the capping station to the printhead face via a rack and pinion mechanism. If the scanning carriage stops at that position, no priming takes place and the printhead remains capped. If the scanning carriage is driven further off the paper, the priming pump plunger, which is integral to the capping station, encounters the machine frame and both the plunger and a return spring are compressed. This action also serves to purge any waste ink from the priming pump chamber but, at this point, no priming of the printhead has occurred. The scanning carriage is then driven back to the capping position, where the cap seal is still in contact with the printhead face, and the spring is allowed to relax and drive the pump piston to perform a controlled prime operation. The scanning carriage remains in this position for a minimum of two seconds to allow an adequate amount of ink to be drawn from the printhead orifices. The excess ink is collected in the capping station volume. It is desirable, therefore, to remove this waste ink from the capping station volume after each prime.